The proportion of digital images requiring printing or display is growing with the increasing use of digital images in the home, office and on the internet. The originating equipment is typically a digital camera or scanner. In the case of a digital camera, the scene illumination may be less than ideal, or there could be high levels of noise in the camera image file that results in noisy and/or colored shadows in the final image. In the case of a print scanner color cast in the original print could give unacceptable colored shadows in the scanned version of the print. Even if the originating equipment provides an accurate ‘well balanced’ image any subsequent processing may introduce color casts or noise.
U.S. Pat. No. 4,160,264 provides a matrixing method to correct for color casts in shadows. However this document provides no method for automatically modifying the point at which the matrixing takes effect for a wide range of images such as under exposed and back lit. Nor does U.S. Pat. No. 4,160,264 provide any additional means to reduce the noise levels in the dark areas of the picture.
There are a number of ‘fixing’ algorithms that attempt to correct for poor quality images, such as ‘Auto-levels’ in Adobe Photo-shop and HP patent U.S. Pat. No. 5,812,286 that pins the ‘black’ and ‘white’ point of each color. These processes generally affect the whole image in order to correct for the errors in the shadow color. Computers are also increasingly using color management and printer profiles to improve the rendition of printed images, but they cannot take into account color cast errors in the original image data.
Digital images, from digital cameras, scanners or any other suitable source can have dark color shadow regions. The invention aims to take any digital image and correct it for color casts or excessive noise in the shadow or dark areas of the image.